Ionomer membranes are used in many processes, for example, in membrane fuel cells, in electrodialysis, in diffusion dialysis, in electrolysis (PEM electrolysis, chlorine alkali electrolysis), or in electrochemical processes.
A disadvantage of the actual membranes is, however, that their proton conductivity at temperatures above 100° C. in most cases decreases rapidly due to drying up of membranes. Temperatures above 100° C. are, however, very interesting for fuel cell applications of ionomer membranes, because above 100° C. the temperature regulation of fuel cells is greatly simplified and the catalysis of the fuel cell reaction is substantially improved (excess voltage decreased, no CO-loading any more, which poisons the catalyst).
Only a few examples of membranes which still exhibit good proton conductivity even above 100° C. are known from the literature, for example poly(phenylene)s having carbonyl-1,4-phenylene-oxyphenyl-4-sulfonic acid side groups. However the proton conductivity of these membranes decreases rapidly above 130° C., and the reason for the good proton conductivity between 100° C. and 130° C. is also not clear.
Proton conductivity is based on the Grotthus mechanism with protons in acidic media and hydroxyl ions in alkaline media acting as charge carriers. There exists a structure crosslinked via hydrogen bonds enabling the actual charge transport. That means the water contained in the membrane plays an important part in the charge transport: without this additional water, there is no mentionable charge transport across the membrane in these commercially available membranes; they lose their function. Other new developments, which use phosphate backbones instead of a fluorohydrocarbon backbone, also need water as an additional network builder. (Alberti et al., SSPC9, Bled, Slowenia, 17–21. Aug. 1998, Extended Abstracts, p. 235). While the addition of small SiO2 particles to the above mentioned membranes (Antonucci et al., SSPC9, Bled, Slowenia, 17–21. Aug. 1998, Extended Abstracts, p. 187) leads to a stabilization of proton conductivity up to 140° C., this only applies under operating conditions of a pressure of 4,5 bar. Without increased operating pressure, these membranes also lose their water network above 100° C. and dry up. A substantial disadvantage of all the above mentioned membrane types is therefore that, even under best operating conditions, they are usable at application temperatures of up to 100° C.
In the same manner as mentioned above, Denton et al. U.S. Pat. No. 6,042,958) prepared composites from ion conducting polymers and porous substrates. As silica containing components, they used glass, ceramics, or silica. In the examples described therein, the operating temperature was not increased above 80° C.
While in the direct methanol fuel cell (DMFC) sufficient water is present, methanol crossover through the membrane, however, results in a substantial decrease of power.
If composites of sulfonated polyaryletheretherketone membranes (European Patent No. EP 0574791 B1) or sulfonated polyethersulfone and silica are prepared, the membrane swells at an cation-exchange capacity of 1.5 meq/g to an extent that it is ultimately destroyed.
Phyllosilicates (clay minerals) have some interesting properties:                They can bind hydrate water up to 250° C.        In these materials, metal cations and metal oxides can be additionally incorporated, inducing hereby an intrinsic proton conductivity according to the general scheme:Mn+(H2O)→(M—OH)(n−1)++H+         [Zeolite, Clay and Heteropoly Acid in Organic Reactions, Y. Izumi, K. Urabe, M. Onaka; 1992; Weinheim, VCH-Verlag, p. 26].        Phyllosilicates having Lewis acid cavities may intercalate by acid-base interaction with the basic groups of basic polymers [Kunststoffhanokomposite, symposium: Von der Invention zur Innovation, Publication at the Symposium of the Fonds of the Chemical Industry, 6th, of May, 1998, in Cologne].        
Due to these properties, some types of phyllosilicate/polymer composites have been synthesized. Muhlhaupt et al made composites from montmorillonite and polypropylene, montmorillonite and polyamide, and montmorillonite and PERSPEX™. In these composites, for example, the PERSPEX becomes hardly flammable, due to the admixture with montmorillonite, because the incorporated phyllosilicates are barriers to the pyrolysis gases formed on combustion.